Apoptosis in proliferating, senescent, and immortalized keratinocytes

Skin provides an attractive organ system for exploring coordinated regulati on of keratinocyte (KC) proliferation, differentiation, senescence, and apo ptosis, Our main objective was to determine whether various types of cell c ycle arrest confer resistance to apoptosis, We postulated that KC cell cycl e and cell death programs are tightly regulated to ensure epidermal homeost asis, In this report, simultaneous expression of cyclin-dependent kinase in hibitors (p15, pig, p21, and p27), a marker of early differentiation (kerat in I), mediators of apoptosis (caspases 3 and 8), and NF-kappa B were analy zed in three types of KCs, By comparing the response of proliferating, sene scent, and immortalized KCs (HaCaT cells) to antiproliferative agents follo wed by UV exposure, we observed: 1) Normal KCs follow different pathways to abrupt cell cycle arrest; 2) KCs undergoing spontaneous replicative senesc ence or confluency predominantly express pie; 3) Abruptly induced growth ar rest, confluency, and senescence pathways are associated with resistance to apoptosis; 4) The death-defying phenotype of KCs does not require early di fferentiation; 5) NF-kappa B is one regulator of resistance to apoptosis; a nd 6) BaCaT cells have undetectable p16 protein (hypermethylation of the pr omoter), dysfunctional NF-kappa B, and diminished capacity to respond to an tiproliferative treatments, and they remain highly sensitive to apoptosis w ith cleavage of caspases 3 and 8. These data indicate that KCs (but not HaC aT cells) undergoing abruptly induced cell cycle arrest or senescence becom e resistant to apoptosis requiring properly regulated activation of NF-kapp a B but not early differentiation.